The invention relates to a process for controlling the heat transfer to a workpiece during vapor phase soldering.
During vapor phase soldering the parts to be soldered are rapidly heated when being immersed into the saturated vapor layer. Re-melting the solder for the purpose of soldering can also take place very rapidly. As a consequence, there are often unpleasant side effects such as, for example, the generation of solder pearls if solder paste is re-melted too rapidly on a printed circuit board. It is therefore desirable to slow down the rapid heat transfer in the vapor phase in order to be able to heat the parts to be soldered more slowly.
It is known that during vapor phase soldering a slow heat transfer can be achieved in that a workpiece with the part to be soldered is allowed to immerse only into the surface area of the vapor phase. Thus, the vapor condenses mainly from the bottom and only to a small extent from the top of the part to be soldered. Normally, the workpiece is held in the upper position of the vapor phase until the solder paste melts slowly. Only then is it further moved down in the vapor phase in order to well heat the soldering joints of all components, so that the solder can well moisten all parts to be soldered.
A difficulty of the known method is, however, that it is true that the heat transfer takes place more slowly but consequently also the soldering process is prolonged considerably. Although the actual soldering process can be over after, e.g., 30 seconds, in the known process a printed circuit board is held for, e.g., 180 seconds in the upper area of the vapor phase in order to be able to re-melt slowly. Thus, problems arise in processes having a critical process time, which can lead to increased costs.
WO 98/41352 discloses a process for heat-treating workpieces, in particular for the hot-connection of electronic components or printed circuit boards with hot vapor in a treatment device comprising at least one vapor generator. In this process the workpieces are heated with vapor in a controllable manner. In this respect, vapor generation is controlled as a function of the amount of heat required to produce the desired temperature gradient in the workpieces.
In contrast thereto, it is an object of the present invention to provide a process for controlling the heat transfer to a workpiece during vapor phase soldering, in which a more rapid heat transfer takes place without the mentioned disadvantageous side effects, and in which the temperature of the workpiece can be adjusted in a narrow temperature range.
The object is achieved with the features of the claims.
In achieving this object, the invention starts out from the basic idea of rapidly heating a workpiece in the vapor phase up to a predetermined temperature usually lying below the melting temperature of the solder and then controlling the temperature by changing the vapor amount above the workpiece, wherein the solder slowly heats up to its melting temperature, preferably by reducing the vapor amount.
The present invention relates in particular also to the heat transfer at the boundary between vapor phase and vapor-free space.
The invention starts out from the fact that the vapor amount generated per time unit is a controlled variable for the heat transfer speed. If this vapor amount per time unit is appropriately dimensioned, the entire vapor volume above the part to be soldered condenses on top of it when this part is immersed into the vapor phase. This leads to a correspondingly rapid temperature increase. On the part to be soldered at most as much vapor can be condensed as the part to be soldered takes up heat per time unit. The vapor amount must be dimensioned such that the amount present corresponds to the amount which can condense when the part to be soldered is introduced into the vapor phase. In this case the vapor volume condenses on the part to be soldered and the upper boundary of the vapor layer is located at the contact level of the part to be soldered. If the temperature of the part to be soldered increases, less vapor can condense on it and the vapor layer rises over the part to be soldered. Thus, vapor can again condense from the top, which leads in turn to a more rapid temperature increase in the part to be soldered. If more vapor is available (or if it is produced in such a short period of time) than can directly condense when the part is immersed into the vapor phase, also the boundary of the vapor layer is located above the part to be soldered, and the maximum heat transfer to the part to be soldered is achieved from all sides. In this case the slow heating process according to the present invention cannot be applied since the delayed heat transfer is based on the fact that during the slow heat transfer the vapor condenses mainly only below the part to be soldered.
The present invention is advantageous in that it provides a rapid and economical process for vapor phase soldering and an exact control of the workpiece temperature.